In the field of long-range radar systems, so-called “phased-array antennas” are part of the state of the art. A special characteristic of these antennas is a plane exterior surface in the viewing direction of the antenna array. Because the viewing angle of this antenna arrangement is azimuthally considerably restricted but a 360° view is required for an operative marine application, this antenna system is periodically rotated about its vertical axis. For an optimal detection of the elevation (total angle of the upper half-space=90°), a basic alignment of the array surface in the range of 45°with respect to the horizontal plane is required in this case.
Based on this background, a problem is detected which occurs in the spectral range of between 0.4 μm-5.0 μm (VIS, NIR, SWIR and MWIR). The plane surface, which forms the outer end of the antenna in the transmitting and receiving direction and which, in the case of an implemented typical antenna, has a surface of 21n m2, represents a considerable reflector for the sun. It is another difficulty that, because of the basic alignment as well as the continuous rotation, the angular condition for specular reflections is met for a wide range of the solar altitude angle as well as of the observer's angle with respect to the sun. A flashing sun reflex of a signal duration of typically approximately 0.3-0.7 s can be observed, which is easily detected by an imaging sensor in the entire spectral range of the solar radiation (VIS, NIR, SWIR and MWIR) and can unfortunately also be classified because of the conceivable low piece numbers of the installed antennas.
In this case, a specular degree of reflection of a few percent will be sufficient for increasing the reflected intensity far above the ambient brightness. For the purpose of a comparison: Depending on the amount of dirt and the orientation, an uncoated plane glass pane reflects 5 to 8% of the incident intensity and, depending on the size, supplies a conspicuous signal over a distance of 20 km for imaging sensors.
The following applies to a typical antenna: Up to an observer's distance of 0.5 km, the width of the antenna fills the entire angular dimension of the sun; at a distance of 10 km still 5% or barely 1 mrad; that is, for most imaging sensors, more than one pixel width. A geometrical “dilution” occurs only at a multiple of this distance; that is, as a rule far behind the horizon. The decisive factor in this case is the large dimension of the almost perfectly plane face of the antenna. A reduction of the spectral degree of reflection (black color) for suppressing the specular reflex would have to be changed to far below 0.5%, which can virtually not be achieved according to the generally known state of the art.
Wide-spread technology would suggest the use of a dull lusterless color. In this case, normally microscopic structures (area 10 μmØ) in the form of pigments or cavities are placed in the surface. However, this is always connected with a high susceptibility to contamination, particularly by means of drops of water. In the surroundings of a battleship, particularly in the proximity of the diesel exhaust gas system, this is very unfavorable; that is, it cannot be maintained for extended time periods because the surfaces have to be cleaned frequently (for example, twice a week), and in this case would lose their dull characteristic just as a result of material ablation.
Furthermore, camouflaging elements are known from German Patent Documents DE 199 55 608 C2 and DE 197 10 692 C2.